nature and trees,” he said. The neuroscience is bomb-proof on
exercise. Physical activity changes the brain to improve memory and
to slow aging; it improves mood and lowers anxiety; in children, it
increases the capacity to learn; some studies show it is as effective as
antidepressants for alleviating mild depression without the unwanted
side effects. By contrast, our collective physical inertia, credited with
1.9 million deaths worldwide annually, is new to our species and
getting worse. In preindustrial times, we expended about 1,000
kilocalories per day on physical activity; now we expend an average
of about 300.
What changed Mitchell’s mind, gradually, was reading the studies
from Japan that showed lower stress among forest walkers but not
city walkers. There were also some studies showing that people who
lived near parks and green areas were healthier, even though they
didn’t necessarily exercise in them. There was something else going
on. And that something else had the potential to make a difference in
people’s lives.
But he still didn’t discount the role of exercise. Time in nature, as
the structure of this book suggests, appears to have a dose curve. Five
minutes is good; a thirty-minute stroll is better. When you combine
exercise and nature, the effects get bigger. “Maybe it’s just additive.
But maybe it’s more than that,” he said. To show me, he invited me to
join him for some rambling, the favorite national pastime, especially
when it involves drinking whiskey.
WE MET AT Mitchell’s walk-up garret of an office on campus, out of
which he runs the Centre for Research on Environment, Society and
Health. Mitchell is wiry and tall, and had to fold himself into his car
for the short drive to the edge of town. We’d be ascending Dumgoyne,
part of a chain of volcanic hills circling the city to the north. Kitted
out with hiking boots, a knapsack filled with “waterproofs” and two
walking poles, he eyed my worn sneakers and array of notebooks,